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Resource Interchange File Format

The Resource Interchange File Format (RIFF) is a generic meta-format for storing data in tagged chunks.

It was introduced in 1991 by Microsoft and IBM, and was presented by Microsoft as the default format for Windows 3.1 multimedia files. It is based on Electronic Arts's Interchange File Format, introduced in 1985, the only difference being that multi-byte integers are in little-endian format, native to the 80x86 processor series used in IBM PCs, rather than the big-endian format native to the 68k processor series used in Amiga and Apple Macintosh computers, where IFF files were heavily used.

(The specification for AIFF, the big-endian analogue of RIFF, was published by Apple Computer in 1988.)

The Microsoft implementation is mostly known through file formats like AVI, ANI and WAV, which use the RIFF meta-format as their basis.

Explanation

RIFF files consist entirely of "chunks". The overall format is identical to IFF, except for the endianness as previously stated, and the different meaning of the chunk names.

All chunks have the following format:
* 4 bytes: an ASCII identifier for this chunk, e.g. "fmt " or "data". * 4 bytes: an unsigned, little-endian 32-bit integer with the length of this chunk (except this field itself and the chunk identifier). * variable-sized field: the chunk data itself, of the size given in the previous field. * a pad byte, if the chunk's length is not even.
Two chunk identifiers, "RIFF" and "LIST", introduce a chunk that can contain subchunks. Their chunk data, after the identifier and length, has the following format:
* 4 bytes: an ASCII identifier for this particular chunk (in the case of the RIFF chunk: for the entire file, such as "AVI " or "WAVE"). * rest of data: subchunks.
The file itself consists of one RIFF chunk, which then can contain further subchunks: hence, the first four bytes of a correctly-formatted RIFF file will spell out "R", "I", "F", "F".
More information about the RIFF format can be found in the Interchange File Format article.
RF64 is a multichannel file format based on RIFF specification, developed by the European Broadcasting Union. It is BWF-compatible and allows file sizes to exceed 4 gigabytes

Use of the INFO chunk
The optional INFO chunk allows RIFF files to be "tagged" with information falling into a number of predefined categories, such as copyright ("ICOP"), comments ("ICMT), artist ("IART"), in a standardised way. These details can be read from a RIFF file even if the rest of the file format is unrecognised. The standard also allows the use of user-defined fields. Programmers intending to use non-standard fields should bear in mind that the same non-standard subchunk ID may be used by different applications in different (and potentially incompatible) ways.

Compatibility Issues

Initial difficulties with MIDI Files
In line with their policy of using .RIFF for all Windows 3.1 "multimedia" files, Microsoft introduced a new variant on the existing MIDI file format used for storing song information to be played on electronic musical instruments. Microsoft's "new" MIDI file format consisted of a standard MIDI file enclosed in a RIFF "wrapper", and had the file extension .RMI. Since the existing MIDIfile format already supported embedded "tagging" information, the advantages to the user of having a new format were not obvious.
The MIDI Manufacturers Association have since embraced the RIFF-based MIDIfile format, and used it as the basis of an "extended midifile" that also includes instrument data in "DLS" format, embedded within the same .RMI file.

INFO chunk placement problems
For cataloguing purposes, the optimal position for the INFO chunk is near the beginning of the file. However, since the INFO chunk is optional, it is often omitted from the detailed specifications of individual file formats, leading to some confusion over the correct position for this chunk within a file.
When dealing with large media files, the expansion or contraction of the INFO chunk during tag-editing can result in the following "data" section of the file having to be read and rewritten back to disk to accomodate the new header size. Since media files can be gigabyes in size, this is a potentially disk-intensive process. One workaround is to "pad out" the leading INFO chunk using dummy data (using a "dummy chunk" or "pad chunk") when the file is created. Later editing can then expand or contract the "dummy" field to keep the total size of the file header constant: an intelligently-written piece of software can then overwrite just the file header when tagging data is changed, without modifying or moving the main body of the file.
Some programs have tried to address the problem by placing the INFO chunk at the end of a media file, after the main body of the file. This has resulted in two different conventions for chunk placement, with the attendant risk that some combinations of software can cause a file's INFO data to be ignored or permanently overwritten during editing. More sophisticated programs will take into account the possibility of "unexpected" chunk placement in files and respond accordingly, for instance, when the audio-editing program Audacity encounters a .WAV file with end-placed INFO data, it will correctly identify and read the data, but on saving, will relocate the INFO chunk back to the file header.
Although CorelDRAW10 nominally uses a RIFF file structure, the program's initial release placed the INFO chunk at the end, so that any embedded preview bitmap would not be displayed under Windows' file manager by default. A "patch" utility supplied with the program fixes this problem.
The article is based on materials from matroska.org, wikipedia.org.
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